Now as i said, there is a progression (this is where the term 'progressive' comes from) of relations of production; slave system, feudalism, capitalism, socialism; which exists between the two manifestations of humans natural social structuring and production relationship (both kinds of communism). The steps which shift one system to another inevitably are driven by conflict, as those who benefit from one system struggle to secure their place and power from those who suffer under it. The thing that differentiates those who benefit from those who dont is how you draw class lines. Inequalities between classes get worse as a system continues, building to inevitable conflict and revolutions.
Some examples of this in action. The feudal era started out with small groups warring with each other over resources. Eventually power was consolidated and such open conflict became something less common, as it was often not profitable or too risky. This resulted in interactions between groups to become more economic in nature. The conflict was still there, but it was not as onesidedly martial. The trade gave the merchant class a boost, and soon people who were not nobles, who were not depending on their distinct legal class, which gave them privileges over all lower classes, began to get quite powerful, even in some cases far more powerful than most of the nobility. The social landscape changed, where you had this class, the beourgoise, resenting the fact that despite all their ambitions, successess, and fortunes, even a bankrupt petty nobleman had special treatment, special legal protections. This is where the liberal idea of legal equality comes from. All those stories you were told growing up which featured 'nobody being above the law!' is a reflection of this. Their ethical sensibility was shaped by the social characteristics of their time. Eventually they cut off a bunch of heads and became the new ruling class. Now we enjoy equal (on paper) laws.
Because the new ruling class were merchants and businessmen who gained their power not through noble blood but through the accumulation of wealth, the nature of the system, capitalism, under them created the industrial revolution as it sought ever more wealth accumulation.
The society capitalism created, however, saw people moving more and more to cities, into great factories and closer living conditions. The social characteristics of this, social living, social working, social production, lead to the sentiment of social ownership as well, socialism, which we saw growing in the 19th and early 20th century because the great wars gave capitalists enough of a boost to dampen the downsides of late stage capitalism long enough that people did not rebel.
As you see, the progression of systems, the effect of the material conditions on society, and the changes society has on its material conditions, create a series of progressive steps. Capitalism was good when it was an advancement over feudalism, but it is bad now for numerous reasons. Socialism is good now, but it too will need to give way to communism one day. Yes, even slavery was good in its time, because it was the step that came after what was before, and before what came after. There is no eternal ethical praise or condemnation, only what makes sense to the society at the time, as it drives ever forward.
Now as far as your first question. When ever people make claims that some identity group is being opressed or has privledge or such, how do they justify these statements? What evidence do they provide? It almost always involves some economic measure. They will prove certain group is having problems because those people, when taken as a group, are averaging less in one thing or another.
Identity politics is a liberal idea, because it effectively embraces the liberals idea that legal equality is equality full stop. This is a reasoning pushed by the rulers under capitalism, so they can say 'we are all equal now so my abundances are just because of my own personal virtues and skill!'. Likewise, to maintain the idea that legal equality is equality, to explain any discrepancies they must invent some sinister force which is actively and destructively sabotaging the system which would clearly result in paradise for all otherwise. There is where ideas of racism, sexism, and other things grow from. The reason there are not fewer female CEOs is because male CEOs are conspiring against them! The reason there are fewer female coders is toxic male environment! The reason there are more blacks in prison is because they are disproportionately targeted!
But what are all these things? The complaint is that classess are skewed, when the real goal is abolishing class entirely! What purpose then is served by focusing on these claims, not even to the point of questioning their validly or accepting them, when it is all moot in the context of obtaining social equality?